Investments - Schedule of Contractual Maturities of Investments (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Amortized cost
Due in one year or less	$ 699
Due after one year through five years	2,690
Due after five years through ten years	3,822
Due after ten years	2,895
Total	10,106	$ 9,799
Fair value
Due in one year or less	714
Due after one year through five years	2,830
Due after five years through ten years	3,970
Due after ten years	2,988
Total	$ 10,502	$ 10,085
% of fair value
Due in one year or less	6.80%
Due after one year through five years	26.90%
Due after five years through ten years	37.80%
Due after ten years	28.50%
Total	100.00%